Little is known concerning the mechanism of action of the polypeptide hormone relaxin. Since most polypeptide hormones exert their biological effects by binding to specific receptor sites on the cell membranes of target tissues, it is proposed to investigate this initial step in the mechanism of action or relaxin. Therefore, one objective of this research proposal is to establish and characterize receptor sites for relaxin in the uterus, cervix, and mammary gland of the rat under different hormonal states of the animal. Binding will be analyzed using I125 labeled relaxin by means of gamma ray spectroscopy and autoradiography. Binding data will be correlated with the hormonal state of the animal. Factors which regulate relaxin synthesis and/or secretion are not known. It is proposed to establish an organ culture system for rat corpora lutea of pregnancy or a rat luteal cell culture. Upon establishment of such a culture system, factors which influence relaxin synthesis and secretion will be investigated by means of ultrastructural studies and immunoprecipitation.